1. Field of the Invention
The present invention relates to process for making a thermally sensitive, negatively-working printing plate having excellent unbaked press chemical resistance.
2. Brief Description of Art
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and other materials. Commonly, the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A widely used type of lithographic printing plate has a radiation-sensitive coating applied to an aluminum base support. The coating may respond to radiation by having the portion that is exposed become soluble so that it is removed in the developing process. Such a plate is referred to in the art as a positive-working printing plate. Conversely, when that portion of the coating that is exposed becomes hardened, the plate is referred to as a negative-working plate. In both instances, the coating areas remaining after development (the image areas) are ink-receptive or oleophilic and the coating areas or background removed development (the non-image areas) are water-receptive or hydrophilic. In the case of direct digital imaging, the plate is exposed to infrared radiation from a laser. The path of the laser is predetermined by a computer program. In the instance of negative-working plates, the areas on the plate corresponding to the image areas are hit by the laser, allowing the infrared radiation to harden the image area coating, while the areas on the plate corresponding to non-image areas are not hit by the laser, thus preventing the radiation-hardening process, so the areas not struck by infrared radiation can be removed during development. The radiation-hardened surfaces of a negative-working plate are therefore oleophilic and will accept ink while the non-image areas that have had the coating removed through the action of a developer are desensitized and are therefore hydrophilic.
Direct digital imaging of offset printing plates is a technology that has assumed increased importance to the printing industry. Successful workings of such technology have used either single lasers or an array of lasers that emit modulated near IR or IR radiation have been used. In particular, high output and compact solid-state laser devices and semiconductor laser devices emitting modulated near infrared or infrared rays of a wavelength of about 750 nm to 1,200 nm have been employed as the radiation source for recording in the direct production of a printing plate from digital computer data. Such laser devices include those using a laser emitting at about 830 nm or at about 1056 nm or about 1112 nm. Commercially available imaging devices include the CREO Trendsetter available from Creo of British Columbia, Canada and the Gerber Crescent 42T available from Gerber of Brussels, Belgium.
Several specific methods of such employing laser technologies in the production of printing plates have been described in the patent literature, including the following:
U.S. Pat. No. 4,544,627 (Takahashi et al. assigned to Fuji Photo Film) discloses a negative image forming process which comprises (1) uniformly exposing a photosensitive material comprising a photosensitive material comprising a support layer having a sensitive layer thereon to actinic radiation; (2) subsequently imagewise exposing that uniformly exposed photosensitive material with a laser beam; and (3) then developing that imagewise exposed photosensitive material with an alkaline developing solution. The sensitive layer comprises (a) an o-quinonediazide component and (b) a second compound which is capable of varying the dissolution rate in the developing solution. The uniform exposure with actinic radiation converts the o-quinonediazide compound to the corresponding indenecarboxylic acid compound. The imagewise exposure of the sensitive layer with the laser beam renders the indenecarboxylic acid compound in the imagewise exposed areas to be converted to the corresponding indene compound. The developing step dissolves out the areas of the sensitive layer that were not imagewise exposed with the laser beam. The presence of the second compound in the sensitive layer reduces the rate of dissolution of the laser exposed areas in the developing solution. This patented process involves the use of a single sensitive layer that is placed directly on the support layer and does not teach or suggest the addition of a intermediate polymer layer between the support layer and the naphthoquinonediazide-containing sensitive layer or the advantages of employing such an intermediate polymer layer.
PCT Published Patent Application No. WO 96/20429 (Bennett et al. xe2x80x94assigned to Horsell P.L.C.) discloses a method of forming a lithographic plate which comprises (1) coating a lithographic base with a positive working photosensitive composition; (2) overall exposing the photosensitive composition on the plate with ultraviolet light to render the photosensitive composition developable; (3) imaging the thus overall exposed photosensitive composition on the plate with a laser; and (4) then developing the plate to remove those areas of the photosensitive composition not exposed to the laser. The photosensitive composition comprises (a) naphthoquinonediazide ester; (b) a phenolic resin and (c) at least one substance which absorbs infrared radiation. This process also involves the use of a single sensitive layer that is placed directly on the lithographic base and does not teach or suggest the addition of a intermediate polymer layer between the support layer and the naphthoquinonediazide-containing sensitive layer or the advantages of employing such an intermediate layer.
European Published Patent Application No. 0 864 420 A1 (Van Damme et al.xe2x80x94assigned to Agfa-Gevaert N.V.) discloses a lithographic process that comprises (1) making a lithographic imaging element having (a) a lithographic base having a hydrophilic surface, (b) a hydrophobic layer placed on the base, said hydrophobic layer comprising a polymer that is soluble or dispersible in an aqueous alkaline solution; and (c) a top layer placed on the hydrophobic layer, said top layer sensitive to IR-radiation; (2) imagewise exposing the element to IR-radiation; and (3) developing the exposed imaging element with an aqueous alkaline developing solution. The top layer comprises an IR-absorbing compound, a binder resin and optionally, a compound sensitive to visible light and/or UV-radiation. The hydrophobic intermediate layer is preferably composed of hydrophobic binders conventionally used in positive or negative working PS-plates, e.g. novolacs, polyvinyl phenols, or carboxy-substituted polymers. This method uses a one-stage exposure process that does not include a UV-flood exposure before the imagewise IR-radiation exposure.
European Published Patent Application No. 0 908 305 A1 (Deroover et al.xe2x80x94assigned to Agfa Gevaert N.V.) discloses a method for making lithographic printing plates that comprises the steps of (1) preparing a heat mode imaging element comprising (a) a lithographic base with a hydrophilic surface; (b) a polymeric first layer on top of the base, said polymeric first layer including a polymer that is soluble in an aqueous alkaline solution; and (c) a top layer on top of the first layer, said top layer including an IR-dye and being sensitive to IR-radiation and is impenetrable for an alkaline developer containing SiO2 as silicate; (2) imagewise exposing this heat mode imaging element to IR-radiation; and (3) developing said imagewise exposed heat mode element with a silicate-containing alkaline developer so that exposed areas of the top layer and the underlying areas of the first layer are dissolved and the unexposed areas of the top layer and the underlying portions thereof remain undissolved. This process also uses a one-stage exposure process that does not include a UV-flood exposure before the imagewise IR-radiation exposure.
European Published Patent Application No. 0 908 307 A2 (Van Damme et al.xe2x80x94assigned to Agfa-Gevaert N.V.) disclosed a method for making positive printing plates that is similar to the method disclosed in EP 0 908 305 A1 except the IR-sensitive top layer includes selected IR-dyes. This method also uses a one-step exposure process that does not include a UV-flood exposure before the imagewise IR-radiation exposure.
European Published Patent Application No. 0 908 779 A1 (Vermeesch et al.xe2x80x94assigned to Agfa-Gevaert A.G.) discloses a method for making positive working printing plates from a heat mode sensitive imaging element that is similar to the method disclosed in EP 0 908 305 A1 except the top layer includes an organic quaternary ammonium salt. This method also employs a one-stage exposure that does not include a UV-flood exposure before the imagewise IR-radiation exposure.
European Published Patent Application No. 0 908 784 A1 (Vermeesch et al.xe2x80x94assigned to Agfa-Gevaert N.V.) also teaches a method of making positive working printing plates from a light sensitive imaging element that is similar to the method of EP 0 908 305 A1 except the top layer includes a diazonium salt and at least 20% of its weight is a non-proteinic hydrophilic film-forming polymer. Again, this method uses a one-stage exposure that does not include a UV-flood exposure before this imagewise IR-radiation exposure.
European Published Patent Application No. 0 908 728 A1 (Verschueren et al. assigned to Agfa-Gevaert N.V.) discloses a method for making lithographic printing plates that is similar to the method disclosed in EP 0 980 305 A1 except that the top layer contains at least one compound containing epoxy units and a hardener. Again, this method uses a one-stage exposure that does not include a UV-flood exposure before the imagewise IR-radiation exposure.
European Published Patent Application No. EP 0 914 941 A1 (Vermeesch et al.xe2x80x94assigned to Agfa-Gevaert N.V.) discloses a method for making positive working printing plates from a heat mode sensitive imaging element that is similar to the method disclosed in EP 0 908 305 A1 except that the alkaline developer used herein has a pH of at least 12 and a surface tension of at least 30 mN/m. Also, Example 3 of this patent application discloses the use of naphthoquinonediazide-containing compound in the top layer. Like the previously referred to Agfa references, this method uses a one-stage exposure that does not employ a UV-flood exposure before the imagewise IR-radiation exposure.
European Published Patent Application No. 0 919 868 A1 (Nakamura et al.xe2x80x94assigned to Fuji Photo Film, Ltd.) discloses a positive-type image forming process which comprises (1) coating a specific imaging-forming material onto a suitable substrate; (2) subjecting the imaging-forming material to imagewise exposure, preferably with a IR-radiation source; (3) optionally, heating the imaged material; and (4) subjecting the heated, imaged material to a developing solution to form a positive image. The image-forming material comprises (a) an aqueous alkaline solution-soluble polymer having a phenolic hydroxy group; (b) a compound that will suppress the aqueous alkaline solubility of the polymer (a), but loses that property when decomposed by light or heat; and (c) a cross-linkable compound that will cross-link with the polymer (a) upon heating and will increase the solubility-suppressing effect of compound (b). One preferred class of compound (b) is naphthoquinone diazide compounds. When this positive-type imaging-forming material is imaged with a laser, the laser-irradiated portion of the material becomes soluble in the developer (i.e. compound (b) loses its xe2x80x9csolubility-suppressing effectxe2x80x9d in those portions) whereas in the non-laser-irradiated portion, this compound (b) still suppresses the aqueous alkaline solubility of that portion. Accordingly, an image is formed on the printing plate by developing due to the differences in the aqueous alkaline solubilities of the two different portions. Again, this reference uses a one-stage exposure that does not employ a UV-flood exposure before the imagewise IR-radiation exposure.
U.S. Pat. No. 5,705,308 (West et al.xe2x80x94assigned to Kodak Polychrome Graphics) discloses a method of forming a positive image through infrared exposure that comprises (1) imagewise exposing with infrared radiation an imaging layer on a support; (2) preferably, flood-UV exposing the imagewise exposed layer; and then (3) developing the imaged layer with an aqueous developing solution to remove the imaged areas of that layer. The imaging layer contains two essential components, namely, an infrared absorbing compound and a phenolic resin that is either mixed or reacted with an o-diazonaphthoquinone compound. These patented methods involve the use of a single imaging layer and do involve a non-imaging polymeric intermediate layer between the support and the imaging layer.
While the above-noted processes have produced commercially acceptable printing plates, it has been found that the new types of fountain solutions now commonly used to wet the non-imaged areas of the plate are more aggressive towards the coating layers than prior types of fountain solutions. When these aggressive fountain solutions are used with conventional single layer coating layers, they can undesirably remove some of the imaged areas of the plate before those areas are inked or they can attack these imaged areas during the printing run, thus reducing the useful lifetime of the plate. To overcome this problem, a post-development heating step has been used to obtain the required chemical resistance of the coating layer to these aggressive fountain solutions. However, such post-development heating steps complicate the printing plate fabrication process and increase the cost of the plates. Accordingly, there is a need to overcome this problem. The present invention overcomes this problem.
Therefore, one aspect of the present invention is directed to a negative-working image forming process which comprises the steps of:
(1) flood exposing with actinic radiation a photosensitive assembly that comprises:
(a) a hydrophilic support which can be used as a lithographic base and having thereon;
(b) a first layer comprising at least one polymer that is soluble or dispersible in an aqueous alkaline solution;
(c) a second layer on top of the first layer, the second layer comprising at least one o-quinonediazide compound; whereby the flood exposure causes the at least one o-quinonediazide compound to be converted to the corresponding indenecarboxylic acid compound; and
(d) at least one photothermal conversion material in either the first layer or second layer or both;
(2) imagewise exposing the flood exposed photosensitive assembly with infrared radiation to thereby convert the indenecarboxylic acid compound in the second layer in the imagewise exposed areas to the corresponding indene compound; and
(3) developing the imagewise exposed photosensitive assembly with an alkaline developing solution to dissolve out the imagewise unexposed area of the second layer and the areas of first layer underlying the imagewise unexposed areas of second layer and thereby producing a negative working image.
The present invention has several commercially significant advantages. A thermally sensitive, negatively working printing plate having excellent press chemical resistance but not requiring baking may be produced because this particular two layer coating structure provides extra protection against the aggressive fountain solutions. Also, this process allows for the use of low pH (i.e., below about 13.5) developers, thereby increasing the window of operation for this process. Finally, because the process is negative-working, the laser used in the imagewise exposure step will last longer than if the process was positive working.